Why did Baba insist that the author stop reading the Bhagavat and go to the Masjid to listen to stories instead?

The events described in Chapter 18 are actually the reverse of what your question suggests. The author was reading the Bhagavat and, on his own, put the book aside to run to the Masjid out of a desire to hear Baba's stories. However, the text clarifies that Baba was not satisfied with the sacred reading being left incomplete. In what the author calls a 'great marvel,' Baba insisted that he return and finish reading the remaining part of the Bhagavat, demonstrating the immense importance Baba placed on the scripture reading.